Cholesterol is a ubiquitous lipid essential for normal human development, growth, and physiology. Cholesterol is present in the retina, which maintains cholesterol homeostasis by balancing cholesterol input (local biosynthesis and tissue uptake from the systemic circulation) and cholesterol output (local metabolism to oxysterols and transport to the systemic circulation by lipoproteins). Elaborate mechanisms control and coordinate retinal cholesterol input and output to maintain steady-state cholesterol levels. Retinal cholesterol has been linked to age-related macular degeneration (AMD), the leading cause of vision loss in the elderly of the industrialized world. The details of the cholesterol-AMD link are, however, unclear, due in part to significant gaps in our knowledge about retinal cholesterol. During the previous grant period, we used human tissues and mice to investigate two aspects of retinal cholesterol homeostasis: 1) local metabolism to oxysterols by cytochrome P450 enzymes CYP27A1 and CYP46A1, and 2) retinal responses to dietary and pharmacologic treatments. Metabolism to oxysterols appeared to be an important contributor to retinal cholesterol output, and a process maintaining the normal status of retinal vasculature. We found that the production of oxysterols is disturbed by light-induced retinal lipid peroxidation but protected in part by mouse treatments with pyridoxamine, a B6 vitamer. Local cholesterol biosynthesis and cholesterol elimination by lipoproteins were the other two cholesterol-related pathways in the retina responsive to pharmacologic treatments. The former was suppressed and the latter was upregulated by the treatments with simvastatin and TO901317, respectively. Simvastatin is a cholesterol lowering drug and TO901317 is an agonist of transcription factors from the family of liver X receptors. Unexpectedly, we identified two proteins, apolipoprotein D (apoD) and acyl-CoA cholesterol acyltransferase 1 (ACAT1) as potentially important for retinal cholesterol transport and storage, respectively. In this renewal we will focus on those unknown aspects of retinal cholesterol input and output that are of unquestionable importance for our understanding of how to combat AMD. The Specific Aims are: 1) to ascertain the contributions of local cholesterol biosynthesis and tissue uptake of blood-borne cholesterol to a pool of retinal cholesterol and whether these contributions are changed by high- cholesterol diet or treatment with simvastatin; 2) to establish retinal significance of apoD and ACAT1 and the pathways of retinal cholesterol transport and storage; 3) to assess the effectiveness of pharmacologic treatments of mice with significant accumulations of retinal cholesterol and abnormalities of retinal vasculature. We will obtain principally new information about retinal cholesterol acquisition, elimination and storage, fill the most critical gaps in our knowledge about retinal cholesterol maintenance, and facilitate the development of pharmacologic treatments for ocular diseases associated with deleterious accumulations of cholesterol in the retina.